As it is well known within the art, liquid hot metal is either poured directly out of the bottom of the blast furnace through a trough into a ladle car for transfer to the steel mill. If it cannot be processed directly, it is cast in the form of ingots, so-called pigs, for storage or for further transport.
Nowadays, so-called pig casting machines or apparatuses are used for manufacturing said pigs of pig iron. They conventionally comprise a casting station, at least one endless conveyor with a plurality of casting molds, as well as a removal station at the discharge point of the endless conveyor.
When reaching the casting station, the (empty) casting mould is filled with liquid hot metal and is conveyed to the discharge point. The conveying path and/or conveying time of the metal within the casting machine must be chosen such that the liquid metal in the respective pig casting mould is essentially solidified before reaching the discharge point. In order to accelerate the solidification process of the liquid hot metal and to thereby reduce the required length of the conveyor, the pig casting machines generally further comprise an active cooling zone at some distance of the casting station and before the discharge point. In this active cooling zone, the casting molds and/or the metal inside are generally cooled with water, either from the sides or beneath the molds or from above, or using any combination thereof. Examples of casting machines with active water cooling zone(s) are described e.g. in U.S. Pat. No. 4,605,055, in JP 4 052050 or in FR 1 302 669. When passing through the discharge point, the casting molds either automatically empty by tilting over an inversion point or the pigs are removed by means of a corresponding device.
During the slow solidification of a Fe-C alloy containing more than 4.3% C (i.e. a hypereutectic composition), such as pig iron, part of the carbon is pushed out of the liquid matrix and forms light, flyable particles named “graphite kish” or “carbon flakes” on the surface of the solidifying metal. This is especially true while casting metal out of a blast furnace (C=4.3%-5.1%). Because of their light weight, these graphite particles can fly everywhere, in and around, the workshop and can be a concern in term of industrial hygiene and environmental protection.
Furthermore, the graphite particles still remaining on the surface of the pigs when entering the active cooling zone in the upper part of (a first section of) the conveyor are lifted off by the steam formed when the cooling water comes in contact with the hot metal which leads to a further dissemination of the graphite particles around the plant.
At least, in theory, the phenomenon of flaking out can be avoided if the solidification speed is sufficient enough to freeze the liquid composition in its initial state. In practice however, this is rarely possible due to the size of the pigs.
Although graphite kish formation is a particular issue during pig iron casting, the dust formation during casting of other metals or slags may also be a concern. The dusts in all these cases may take their origin in the smelting of the metal itself, in the pouring of the molten metal in the casting station or during reaction with the ambient atmosphere, such as slag and dross residues, fine solidified particles of metal and metal oxides, etc.